Everyone knows that having women on a company’s board of directors is good for the company’s bottom line. (If you need a refresher, click on the business case, a free download on my website.) A Catalyst study indicates that what makes a difference to the bottom line is not having a woman but having women — plural. A 2006 study concluded that “a critical mass of three or more women can cause a fundamental change in the boardroom and enhance corporate governance.” When there are three, “women are no longer seen as outsiders and are able to influence the content and process of board discussions more substantially.”

This makes sense. If there is one woman, she may not speak up, she may be talked over, or her ideas may be credited to a man who repeats them. As a result, the feminine voice will not be represented. There is no real gender diversity. There is just a token.

When there is a critical mass of women, there is a tipping point. Less likely to feel like outsiders, they are more likely to speak up and be heard. Feminine strengths can be expressed. This is what explains the better results — having a balance of feminine and masculine strengths. Good decisions and good results follow.

If business leaders understood this, you would not expect them to be satisfied with one token woman — or even two. Yet many companies seem satisfied with having a single woman on the board.

There is evidence that “one” is becoming an acceptable norm for women in senior management roles. Recently reported research shows that, when a woman achieves one of the five highest-paying executive jobs in the S&P 500, “the chances of another woman joining the executive team are a whopping 51% lower.” [C]ompanies with top women executives typically had just one.” (This “negative spillover effect” is not caused by the women who get to the top; it is less likely to happen in firms led by women CEO’s.) The research suggests that “firms might be operating with ‘implicit quotas’ in mind.” Having one senior woman, they have “checked the box” on gender diversity objectives.

Why would leaders impose an implicit quota? Is it simply that, after one woman makes it to the top, leaders relax their gender diversity efforts? Is it the unenlightened thought that, if women succeed, men will lose? Is it that leaders do not believe the research showing the value of gender diversity? Is it simply unconscious gender bias? Is it fear of change? We need to understand the underlying cause – so we can understand what it will take for business leaders to drop these “quotas”!